Periodontal disease is the most common oral disorder in the elderly population and showing an increasing incidence with age. It consists of several disorders of the periodontium (tissues surrounding the teeth), including gingivitis and periodontitis, which are chronic infectious diseases. The presence of certain bacteria initiate plaque and calculus formation supra- and subgingival (above and under the gums), which affects the soft tissues (gums) surrounding the teeth and causes inflammation. This gingivitis (inflamed gums) associated with bleeding of the gums, can progress to periodontitis when soft tissue attachment loss and/or supporting bone loss is seen. This bone loss (resulting in pocket formation between the teeth and the remaining soft tissues or bone) may affect one to all teeth and eventually, when left untreated, lead to tooth loss. Furthermore, bacteria in the plaque may cause caries reaching the pulpal tissues of the teeth and cause periapical (around the apex or root point of the tooth) inflammation with periapical bone loss.
Many studies have shown the importance of early detection of periodontal disease, in relation to the prevention of tooth loss and/or the patient's general health. The amount of bone loss detected will determine the treatment choice, ranging from a more conservative approach to drastic extraction of a “lost” tooth. Diagnostic tools are therefore crucial for accurate assessment of the periodontal status. Currently, intra-oral projection radiographs are used to radiographically assess periodontal bone loss. However, these radiographs project all periodontal structures on top of each other, which make differentiation of buccal and oral bone hardly impossible.
In addition, if the x-ray's are not pointed perpendicular to the receptor, bone levels can be misinterpreted because of projection errors.
Thus, one of the main drawbacks of intra-oral radiography is overlap of anatomical structures and lack of three-dimensional (3D) information. This often hinders a true distinction between the buccal and lingual cortical plate and complicates the evaluation of periodontal bone defects, especially the infrabony lesions, also denoted as craters, and furcation involvements (bone loss at the furcation). Depending on the pattern of these infrabony defects, different treatment methods are at hand. For instance, when a local crater has enough supporting bony walls, a membrane can be attached onto its surrounding walls, while bone or bone alternatives are condensed into the cavity of the crater, with as goal the remodelling of this new bone with disappearing or shrinking of the crater (=bone gain). Another example is rootplaning (deep cleaning of roots of the teeth with instruments) to stop further bone loss. To determine the result of these therapies, the current 2D projection radiographs form an additional problem. Using digital radiographs, a radiograph before and 6 months after therapy can be taken and subtracted from one another to see small radiographic density changes. Unfortunately, it is critical to use the same projection angle at both moments, so individualized bite blocks need to be fabricated per patient to follow up the bone. Also, this digital subtraction radiography (DSR) does not provide any 3D information.
The same principle applies when investigating bone lesions around the root end (apex) of a tooth, where 2D projection radiographs often causes misdiagnosis since the buccal and oral bony plates may project onto these local lesions inside the bone. Therefore, recent studies have examined the use of 3D imaging modalities for periodontal diagnosis, as described by Vandenberghe et al. in “Detection of periodontal bone loss using digital intra-oral and CT images: an in-vitro assessment of bony and/or infrabony defects” in Dentomaxillofac Radiol. 2008 (37) 252-260. Besides traditional medical CT images, novel 3D cone beam computed tomography (CBCT) images which are low dose CT images, have recently been introduced in the dental field. It has been shown that CBCT helps in diagnosis of crater and furcation problems and therefore may help in the treatment follow-up of these periodontal defects. When periodontal disease progresses, severe bone loss can lead to tooth loss. Oral implants as replacement-therapy for one or more teeth has become a very predictable surgical procedure and is therefore now routinely used in the modern dental practice. One of the classic criteria for measuring the success-rate of implant-therapy is a local bone loss smaller then 0.2 mm per year, after the first year (Albrektsson et al. in Dent. Clin. North Am. 1986 (30) 151-174). This bone loss therefore needs a precise radiographic follow-up using an individualized set-up, existing of aiming devices and bite-blocks, just like periodontal defects around teeth. Although the critical importance of reducing projection errors with two-dimensional radiographs and using an optimal radiographic protocol, the current literature describing radiographic follow-up in details is sparse. Inter- and intra-observer variability is seldom explored and new three-dimensional techniques are still not commonly used. On 2D projection radiographs, it is namely only possible to measure the vertical bone height (for bone loss assessment), while the bone volume and its remodelling constitutes a three-dimensional process. This indicates that the previously mentioned criteria should be updated.
The same problem is seen when using bone augmentation techniques (bone grafting and ridge preservation). Implant-therapy has several obstacles that can influence its use or success-rate. Beside medico-social factors and the associated bone quality of the patient, the therapy choice is strongly dependent of the remaining bone quantity or the available bone volume at the implant-site. If insufficient bone volume remains, the surgeon can nowadays choose out of many techniques, to prepare the site for implant therapy. These so called bone augmentation techniques can be subdivided into many categories, for instance based upon the recipient site (sinus floor or alveolar ridge) or based upon its complexity (from alveolar ridge preservation techniques after tooth extraction to interpositional bone grafts). In addition, the bone material itself can be autogenous, allogenic, xenogenic (usually bovine) bone or replaced by alloplastic materials for bone growth, with or without combination with membranes for guided bone regeneration and/or BMPs (bone promoting proteins) and PRPs (platelet rich plasma). The success-rate of all these techniques has often been described in the literature, following-up the clinical and radiographic parameters like complications, augmentation failure and implant failure, etc . . . but contradictory results are often demonstrated. This indicates that the choice of augmentation and implant therapy is very complex, and that in addition it is complicated by the continuous technological innovation—on implant as well as bone augmentation level. Also here, the criterion bone loss needs to be put in the context of three dimensions, where bone loss and its remodelling happen in 3D. Again, the current methodology to determine success-rate for these techniques should be questioned. For instance, while Meijndert et al in Clin. Oral Implants Res. 2008 (19) 1295 utilise 2D projection radiographs to determine bone loss and success, Pieri et al in J. Periodontol. 2008 (79) 2093 use 2D measurements on sectional CT slices, both techniques having many limitations.
The recent introduction of low dose three-dimensional imaging has drastically changed the face of dentistry. Low dose cone beam computed tomography (CBCT) has many advantages compared to its big brother, the conventional multi-slice CT scanner, which opened the door to many more clinical applications, as described in Vandenberghe et al. in Eur. Radio. 2010 (20) 2637. Unfortunately, volumetric quantification of bone still has only been employed a few times and rarely for volumetric follow-up of bone loss. Agbaje et al. describe in “Bone healing after dental extractions in irradiated patients: a pilot study on a novel technique for volume assessment of healing tooth sockets” published online in Clin. Oral. Investig. (2008), the use of volumetric quantification for volumetric follow-up of bone loss. The morphology or 3D topography of periodontal defects can be determined using 3D imaging, but also here periodontal disease is a dynamic 3D process that manifests as infrabony craterlike bone loss of periodontal bone surrounding a tooth, which needs follow-up in time. The same situation is seen here, where crater therapy using membranes or bone materials should be followed-up and validated in 3D in stead of on 2D images. Often widths and heights are measured on 2D slices, as e.g. discussed in Chen et al. J. Periodontal 2008 (79) 401, which is better than on projection radiographs, but still this shows a severe limitation since all the above bone remodelling processes are in three dimensions. Agbaje et al. (2008) demonstrated that CBCT is an ideal imaging modality for volumetric determination of the healing of extraction sockets. But one needs also to explore the exact volume determination method or segmentation method more thoroughly. Most dentomaxillofacial segmentation studies are based on manually drawing or coloring the desired structures on each slice in the 3D space. This is very cumbersome. Determination of the border of a structure can be done in many different ways and is dependent of several factors, as e.g. described in Suri et al. Handbook of biomedical image analysis, Volume II: Segmentation models, Springer (2005) 111-182. For instance, manual slice by slice segmentation of structures can be very user dependent and time consuming, especially when two volumes in time need to be processed. Also the 3D scanning of the volume of interest (for instance the head) is usually much larger than the actual region of interest (ROI) (for instance one tooth). Especially the segmentation of bone grafts can be quite difficult because of limited visibility on (CB)CT volumes, as indicated for example in Beaman et al. In Radiographics 2006 (26) 373.
For bone augmentation techniques, implant follow-up or periodontal bone loss follow-up, current literature only uses clinical measurements (width and/or height) on 2D projection radiographs or 2D slices of a 3D volume. Only a few attempts were taken towards volumetric determination using conventional CT, as described in Beaman et al. In Radiographics (2006) 26 p373 and Feichtinger et al. In Cleft. Palate Craniofac. J. (2007) 44 p142, but non using low dose CBCT. Feichtinger et al (2007) follow-up bone volumes but they use a commercial segmentation package (and manually segment out or delineate manually slice-by-slice the bone) without further deepening of the methodology or variables used. Current literature therefore lacks in two main aspects: follow-up of bone volumes and clinical relevance of the segmentation method used. Without an accurate, efficient and universal volume determination method, the optimal bone grafting technique for implant therapy can not be investigated. It thus becomes difficult using the existing methods for answering for example the question which bone graft method of a given selection results in less bone loss. The same goes for implant therapy: which implant results in less bone loss? Or even for periodontal therapy: which technique of crater therapy is more beneficial? Therefore, there is a need for a method for quantification of bone loss (or changes) and implementation on all these clinical indications solving one or more of the above problems.
Current diagnostic approaches have been broadened from 2D projection radiographs to 3D (CB)CT volumes (which are stacks of 2D slices), especially in implant dentistry. However, this bone remodelling is a dynamics process over time, which needs an accurate follow-up. Until now, current follow-up of the changes of alveolar bone is done on 2D radiographs or 2D slices.
A first class of solutions relates to bone follow-up on 2D radiographs. FIG. 1 is a schematic representation of two radiographs at time 1 and 2. Currently, the two ways of measuring bone loss around a tooth or dental implant on 2D projection radiographs is through bone level measurements on radiograph 1 and 2 (double arrows in FIG. 4) or through digital subtraction radiography (DSR) when digital sensors are used and DSR software is at hand.
In addition to the fact that this is only a part of the bone loss that can be assessed (height loss at 1 point while the volume should be assessed), projection errors may project the bone at differently than the actual situation. Also, radiographs at time 1 and 2 need to have almost equal projection geometry to be able to subtract or even compare both radiographs. Individual bite blocks (waxed imprints of the region of interest's teeth) can be fabricated and mounted onto aiming devices for the intra-oral radiographs.
A second class of solutions relates to bone follow-up based on 2D slices of a 3D volume. FIG. 2 is a schematic representation of two computed tomography (CT) scans at time 1 and 2. Notice that the second scan can have a different coordinate system than the first scan.
When measuring bone loss on 2D slices of a 3D volume, a good standardization is still necessary where certain anatomical markers need to be indicated in order to compare measurements on scan 1 and 2. For instance, one can measure the bone loss from the cemento-enamel junction (junction of the crown of the tooth and the root) at the extreme left and right side of the tooth until the alveolar crest. Measurements can now be done on adjacent slices to see if there is a difference in bone loss through the whole bone volume surrounding the tooth. In addition, horizontal measurements can now also be made, where the width (arrows in FIG. 2) of the alveolar ridge can be measured, which is not possible on a 2D projection. The major problem here is that a lot of measurements per tooth need to happen, with again the precise standardization required. This makes assessment of the whole volume almost impossible. Therefore, in current research, measurements are chosen at certain levels (width and height). Again, this results in insufficient volume information for follow-up and it is rather time consuming.
Because of all these limitations it is also very difficult to validate periodontal therapies—from rootplaning to bone augmentation and implant therapy. Which methods results in the least bone loss? This makes it of course also very difficult to precisely follow-up periodontal patients.
Ideally, the whole volume is measured to follow-up bone loss. This volume determination or segmentation can be done slice by slice by selecting the desired region (region of interest, ROI) on both scans (see FIG. 3).
In this scenario, an experienced observer can use the computer mouse to manually trace the region of interest in each slice. Drawbacks are of course the intensive labour required, operator bias and difficulty in obtaining reproducible results. In this way, volume 1 and 2 will both be influenced by these limitations, resulting in inadequate follow-up of bone volumes.